The present invention concerns devices for access and tracking of tracks on the surface of mobile data media readable and/or writable optically by a laser beam.
In the case of a medium in the form of a disk, which shall be taken as the example for the rest of the account, the data is recorded along tracks in the form of either a single spiral extending from the center to the edge of the disk, or a series of concentric circles.
During reading or recording, the disk is in motion. Its movement is rotational about an axis which is perpendicular to the surface of the disk and passes through its center.
Generally, a reading and/or recording device includes means which allow the laser beam to access the track which is to be read and/or written and means to assure the tracking. Tracking is carried out by monitoring and correcting firstly the focussing of the luminous spot that the laser beam makes on the surface of the disk and secondly the radial positioning error of this spot in relation to the axis of the track.
Thus, a classical device for access and tracking includes among other things an access carriage??, a focussing lens and a radial tracking mirror. These elements make up the optical head which is known as the "mobile head". When the device for reading and recording is in operation, the focussing lens and the radial tracking mirror follow a track, whatever the position of the optical head. When the optical head moves the radial tracking stops but the focussing continues. During an access, the access carriage allows firstly the focussing lens and the radial tracking mirror to be positioned above the track, and secondly, the radial tracking mirror is activated to adjust the tracking.
To carry out the access and radial tracking operations, a device in which the motor coil assures both access and radial tracking has been described in the French Patent N.degree. 82 03425.
According to this patent, the optical head is attached to and slides on a guide rail. Means for flexible links between the gliding elements and the optical head are provided in order to obtain a good operation. The use of simple gliding systems does not enable satisfactory tracking to be obtained. Furthermore, in the French Patent N.degree. 82 03425, the focussing lens and its associated coil are linked to the rest of the optical head, which also contains the radial tracking mirror and the access coil, by means of a flexible link in order to avoid the focussing operation being accompanied with a diaphragm effect.
Such a structure for a device for access and control increases the mass of the mobile head and therefore the mass of counterweight necessary to balance it, which has as the main disadvantage of increasing the weight of the head and thereby limiting the speed of access.
By carrying out studies on devices for access and control for an optical disk, the applicant noticed that by using a monobloc structure including the optical head and the two coils for the motor-devices it is possible as is described in French Patent N.degree. 89 08800 to eliminate the means of flexible linkage while maintaining a high level of precision and thus to lighten the mobile head and enable faster access to the tracks.
However, the fact that the optical head and the two coils for the motor-coils are solidly linked is a disadvantage in some cases. When the control device is activated in order to correct the focussing of the laser beam, its movement causes a slight decentering of the beam on the pupil of the focussing lens. For example, when the mobile data medium used has an insufficiently flat surface, this decentering results in deformation and enlargement of the focussing spot accompanied by a reduction in the density of the optical power of this spot. This enlargement may be enough to prevent the laser beam from correctly reading and/or writing data.